A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a “mask” or a “reticle,” may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g., comprising part of, one, or several dies) on a substrate (e.g., a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In order to monitor the lithographic process, it is necessary to measure parameters of the patterned substrate such as, for example, the overlay error between successive layers formed in or on it and critical linewidths in a developed metrology target. There are various techniques for making measurements of the microscopic structures formed in lithographic processes, including the use of scanning electron microscopes and various specialized tools. A fast and non-invasive form of specialized inspection tool is a scatterometer in which a beam of radiation is directed onto a target on the surface of the substrate and properties of the scattered or reflected beam are measured. By comparing the properties of the beam before and after it has been reflected or scattered by the substrate, the properties of the substrate can be determined. This can be done, for example, by comparing the reflected beam with data stored in a library of known measurements associated with known substrate properties. Two main types of scatterometers are known. Spectroscopic scatterometers direct a broadband radiation beam onto the substrate and measure the spectrum (intensity as a function of wavelength) of the radiation scattered into a particular narrow angular range. Angularly-resolved scatterometers use a monochromatic radiation beam and measure the intensity of the scattered radiation as a function of angle.
When a lithography system is first installed it must be calibrated to ensure optimal operation. However, over time, system performance parameters will drift. The key system performance parameters which are subject to drift are the overlay and focus stability of the lithographic apparatus.
A small amount of drift can be tolerated, but too much drift means that the system will likely not meet specification. This problem increases with smaller chip features. As chip features decrease, so do the tolerances, or the process window, that manufacturers must work to. In other words, the smaller the process window the harder it is to manufacture operational chips.
Therefore, wafer manufacturers who use lithography apparatus need to periodically stop production for recalibration of the lithography apparatus. Calibrating the system more frequently can yield a larger process window, but also means more scheduled down time.
In order to reduce the frequency at which production must be stopped to perform calibrations, it is known to perform additional calibrations on the basis of standard measurements retrieved from a monitor or a reference wafer. The monitor wafer can be exposed using a specialized reticle containing special scatterometry marks. From the measurements, it can be determined how far the system performance parameters have drifted from their ideal performance levels and wafer level overlay and focus correction sets can be calculated. These correction sets can then be converted into specific corrections for each exposure on subsequent production wafers.
This off-tool method, using monitor wafers, can be performed on a more regular basis than the full scale calibrations such as, for example, on a daily basis or every other day, without having to stop production. These more regular corrections enable compliance with a narrower process window and improvement in system parameter stability.
Further efficiencies can be realized by using a golden scanner grid (i.e., a map of overlay errors) as the baseline for measuring overlay stability, instead of using random monitoring wafers, meaning that overlay grid matching and long term stability can be achieved in one automated step.
However, even given the benefits of making these types of corrections, there are some problems. The reference wafers are re-used and thus are subject to the effects of aging. An etched grid can be reused for only 20-30 reworks before the monitor wafers need to be replaced, which can have a negative impact on the work in progress. The initial grid quality governs the accuracy of the correction process and this deteriorates with the age of the monitor wafers.